NMR studies of strong hydrogen bond to fluoride ion

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Abstract

Systems such as MF/diol (M = alkali metal) and }1F/carboxylic acid were
subjected to IH, I9F and 13C nmr study to investigate the nature of the
very strong H-bonding of fluoride ions with these systems. Evidence
indicates a strong H-bond in diol-fluoride systems (~H ~ -(56) kJ mol-I)
which is stronger than most 'typical' H-bonds (~H = -(12-40) kJ mol-I),
but weaker than that reported for carboxylic acid-fluoride systems
(~H ~ -(120) kJ mol-I). Approximate fluoride H-bonded shifts (o(OH)OHF)
were evaluated for MF/diol systems from IH chemical shift measurements.
No direct correlation was observed between I9F chemical shift and H-bond
strength. Thermodynamic parameters were calculated from temperature
dependent IH and 19F shifts. Preliminary studies of BUn 4NF-acetylacetone
by I9F nmr were conducted at low temperatures and a possible Jmax (ca.
400 Hz) is reported for the fluoride ion H-bonded to acetylacetone.
Highfield shift for non-protonated carbons and downfield shift for
protonated carbons were observed in carboxylic acid/KF systems. Significant
decreas$in I3C TI due to strong H-bonding to fluoride ions were also
detected in both diol and carboxylic acid systems. Anomalous results were
obtained, such as increasing NOE with increasing temperature in neat
1,2-ethanediol (values above the theoretical maximum of 1.988) and in
1,2-ethanediol/KF. The large 13C NOE's for carboxy carbons in neat
carboxylic acids which are. further enhanced by the addition of KF are also
unusual.